1. Field of the Invention
This invention relates generally to the switching power converters, and more particularly, to circuits and methods for controlling the switching of such converters.
2. Description of the Related Art
Switching power converters can be configured using a number of different topologies, and can be controlled with a wide variety of control methods. For example, a switching converter can be configured as a buck, boost, or flyback converter, and may be operated in discontinuous or continuous inductor current modes. Known control methods include direct duty cycle control, voltage feedforward, and current mode. A discussion of known switching converter topologies and control methods can be found, for example, in “Switching Power Supply Topology Review”, L. Dixon Jr., Unitrode Corp., pp. 1-1 to 1-12 (2001).
Regulation of a switching converter is generally accomplished by varying the duty cycle (D) of the signal operating the converter's switching element. In the case of a buck converter, for example, the output voltage Vout is given by D times the input voltage Vin provided to the converter; thus, Vout tracks Vin unless D is adjusted, with Vout=(1/D)*Vin.
The typical voltage-to-duty ratio converter compares a control voltage to an invariant ramp so that D is linearly proportional to the applied voltage. In the case of a buck converter, this results in linear control overall, but the control voltage must be changed to accommodate changes in input voltage. In the case of a boost converter using the same sort of voltage-to-duty ratio converter, the output voltage varies hyperbolically with the input to the duty ratio controller.
Feedback is usually employed to stabilize the output voltage of either type converter. However, there is a conflict between the desire for high loop gain to reduce errors, and the necessity to limit and control loop gain to assure stability in the frequency domain. This conflict is made more difficult to resolve by the gain which must be added to accommodate a varying input voltage in order to preserve a predetermined error. Moreover, in the case of a boost converter, the loop gain varies hyperbolically with the duty ratio control voltage, due to the 1/D dependence of the output on D.
A technique commonly called “feedforward” can be used to modify the response of the typical linear voltage-to-D controller, but the more serious hyperbolic gain problem in the boost converter is not so addressed and is generally dealt with by reducing the overall gain to make the loop stable at the highest gain level, while permitting more error at the lowest level.